1. Field of the Invention
The present invention generally relates to coated steels having atmospheric corrosion resistance. More specifically, the present invention relates especially to steel that can relatively quickly form an extremely high protective rust layer (hereinafter referred to a high corrosion-resistant rust layer) on a surface of the steel in an atmospherically corrosive environment, the high corrosion-resistant rust layer being effective even in an intensely corrosive environment such as one with a relatively high amount of airborne sea salt.
2. Background Information
Commonly, corrosion resistance of steel in the atmosphere can be improved by adding a small amount of an element such as P, Cu, Cr, and/or Ni. These low alloy steels are called weather resistant steels and within several years of being exposed form a protective rust which can protect against corrosion (hereinafter referred to protective rust). They are what we call maintenance-free steels and do not need corrosion-resistant treatments like painting after rust forms.
However, a relatively long period of time, usually more than 10 years, is required until a protective rust layer has formed. Prior to the formation of the protective rust layer, a float rust and flow rust, such as red and yellow rusts will most likely have formed by initial corrosion. Those rusts are undesirable in appearance and also cause damage by severely decreasing the thickness of steel plates by corrosion. In particular, in an environment that includes airborne grains of sea salt, the protective rust layer is not able to form regardless of the amount of time the steel is exposed. The ability of this protective rust to protect steel deteriorates when atmospheric salinity is rather large. Further, the maximum limit of atmospheric salinity that is allowed during the formation of a high corrosion-resistant rust layer is extremely low.
Concerning this problem, a surface treatment method forming a phosphate film has been proposed and disclosed in JP. Pat. No.01-142088. However, the treatment procedure is complicated as, for example, it requires an appropriate pretreatment before forming a phosphate film. In addition, it is not easy to perform the treatment during welding. Thus, there are difficulties in the application of this method to architectural structures and the like.
Meanwhile, a method of forming protective rust while inhibiting initial corrosion by a resin film was proposed and disclosed in JP. Pat. No.53-022530. However, it has been proven that in an intensely corrosive environment, initial corrosion is not sufficiently inhibited and promotion of the forming protective rust deteriorates by this method. On the other hand, a method using a two-layered film was disclosed in JP. Pat. No.56-033911. By this method, initial corrosion can be controlled but its corrosion resistant ability is so high that a quite long period of time that must elapse before a protective rust forms. Additionally, this method is problematic because it is not easily executed.
All of the methods disclosed in the prior art have difficulty in forming a protective rust in a severely corrosive environment, especially in an environment that includes a relatively high amount of atmospheric salinity. Further, the corrosion resistance of these methods is insufficient.
In view of the above, there exists a need for a coated steel having an ability to form a high corrosion-resistant rust layer in an early stage even in a severely corrosive environment which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
An object of the present invention is to provide a coated steel having an ability to form a high corrosion-resistant rust layer in an early stage even in a severely corrosive environment by applying a surface treatment that is economical and easy to apply onto a surface of a weather resistant steel, normal steel or the like or a surface of a rust layer over these steels.
A first aspect of the present invention is to provide a coated steel with atmospheric corrosion resistance. The coated steel has a steel body and an organic paint layer that coats the body. The layer includes 0.1-30% by weight of calcium oxide, and 0.2 to 60% by weight of a combination of metal sulfate(s) and phosphoric acid. The metal sulfate has a solubility of 1 g or more in 100 g of water at 5 degrees centigrade. The layer is between 1 xcexcm and 200 xcexcm in thickness.
A second aspect of the present invention is to provide a coated steel with atmospheric corrosion resistance. The steel has a steel body, and an organic paint layer that coats the body. The layer includes 0.1-30% by weight of calcium oxide, and 0.2 to 60% by weight of metal sulfate(s). The metal sulfate has a solubility of 1 g or more in 100 g of water a 5 degrees centigrade. The layer is between 1 xcexcm and 200 xcexcm in thickness.
A third aspect of the present invention is to provide a coated steel having atmospheric corrosion resistance. The coated steel has a steel body and an organic paint layer. The organic paint layer coats the steel body. The layer includes 0.1-30% by weight of calcium oxide, and 0.2 to 60% by weight of phosphoric acid. The layer is between 1 xcexcm and 200 xcexcm in thickness.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.